This invention relates to a method and apparatus for generating electric power by means of a windmill in which a generator that is equipped with a rectifier feeds direct current to a common d.c. interconnecting grid. More specifically this invention relates to a windmill system in which two or more windmills supply d.c. power to a common d.c. interconnecting grid which in turn delivers the d.c. power to a converter that transforms the electrical power to alternating current that is then output onto an a.c. grid.
U.S. Pat. No. 5,225,712 describes a method for supplying electrical power from one or more generators driven by a windmill to an a.c. grid. The a.c. power from the generators is converted to d.c. power and then applied to the a.c. grid through inverters. There is no common d.c. interconnecting grid described in the ""712 patent and onto which the d.c. power from the respective generators is supplied.
In the ""712 patent the level of the d.c. voltages developed from the generators is constant and independent of the speed of rotation of the generator or the wind speed. The efficiency of the generator at the various speeds of rotation is affected, inter alia, by the voltage output by the generator, which means that for a single setting of the level of the direct current the efficiency of the generator is unnecessarily low either at a high speed of rotation and wind speed or at a low speed of rotation and wind speed, unless the generator is fitted with special and expensive provisions to improve efficiency.
With a method and apparatus of the invention the d.c. voltage of a d.c. interconnecting grid is adjusted according to circumstances whereby the generator of the windmill is able to operate at a higher efficiency over its entire working range and can be of simpler design. Hence, with the ability to vary the voltage level of the d.c. interconnecting grid to, for example tailor it to the power being supplied by the generator, control over the various windmills used in a wind farm can be greatly simplified. This arises because a reduction in the d.c. voltage of the interconnecting grid enables the generator to also supply a lower voltage and deliver its power with a lower speed of rotation of the windmill and with a higher efficiency.
The adjustability of the voltage of the d.c. interconnecting grid enables the generator to deliver electrical power on the basis of wind speed and thus maximizes efficiency at all wind speeds. With the voltage of the interconnecting grid set on the basis of wind speeds encountered by the windmills and the speeds of rotation of the respective generators. This is particularly advantageous when the windmills are spaced far apart and/or local condition vary rapidly.
Cost savings can be obtained with a method and system of this invention by drawing electrical power for driving local equipment at the respective windmills from the same or common electrical cable of the d.c. interconnecting grid. This electrical power can then be transformed into a.c. when such power is required by local equipment.
Special components and techniques can be advantageously provided to intermittently store energy to enable one to safely stop a windmill when this is needed. Use of a common d.c. interconnecting rid enables a convenient source of electrical power from the a.c. output grid with the use of a power converter to startup the windmills in the farm, for example during a lull in the wind.
The invention is explained hereinafter with reference to a particular embodiment with the aid of an embodiment shown in the drawings.